1. Field of the Invention
The present invention relates in general to an organic electroluminescent panel and, more particularly, to a pixel structure of an organic electroluminescent panel and a method for fabricating the same for reducing cost and increasing the panel quality.
2. Description of the Related Art
Compared with other flat panel display technologies, organic electroluminescent panels have many preferred advantages such as self-luminescence, high brightness, wide viewing angle, high contrast, low power consumption, high speed response, widely temperature operating range, high luminescence efficiency, and simple fabrication.
The conventional organic electroluminescent device has a multi-layer structure mainly including an organic emission layer between an anode and a cathode for generating electroluminescence. A hole injection layer and a hole transport layer are formed between the organic emission layer and the anode. An electron transport layer is formed between the organic emission layer and the cathode. Generally, an organic electroluminescent panel is constituted of red light (R), green light (G), and blue light (B) sub-pixels. A pixel includes at least one red light sub-pixel, one green light sub-pixel, and one blue light sub-pixel. Generally, the arrangement of the RGB sub-pixels of the organic electroluminescent panel continues using the arrangement of the RGB sub-pixels of the liquid crystal display panel. The common arrangement of the RGB sub-pixels includes stripe arrangement, mosaic arrangement, and delta arrangement (triangle arrangement), and the stripe arrangement is the most commonly used. Because the RGB sub-pixels for full-color OLED display are usually fabricated by shadow mask evaporation, there are many disadvantages in the fabrication process. The disadvantages include:
1. Higher cost: All kinds of equipments such as mask fabrication, a precise alignment system, a mask chamber, a mask cleaning system, and mask inspection equipment are needed for the fabrication process. These equipments increase the manufacturing cost for even more than one hundred million New Taiwan Dollars.
2. Lower yield: In the mask alignment process due to misalignment of the mask, particle adhesion, or pressing damage decreases the yield of the panel.
3. Lower resolution of the panel: The opening of each mask is usually adaptive to correspond to the evaporation area of one sub-pixel. Thus, the area of the opening of the mask determines the resolution of the organic electroluminescent panel, and the resolution of the conventional full-color panel is limited to the process capability. The resolution of the conventional panel adopting the shadow mask process is approximately between 120 and 150 ppi (pixel per inch) and is usually not more than 180 ppi.
4. Lower production flexibility: Fabricating the shadow mask generally needs about 6 weeks, and only one pattern is allowed to be formed on one shadow mask to prevent non-uniform tension. Thus, the shadow mask fabrication has only limited flexibility.
Many manufacture companies provide mask-free organic electroluminescent panels (for example, U.S. Pat. No. 5,294,869 and U.S. Pat. No. 6,517,996). However, these conventional technologies have some common disadvantages. That is, the original RGB device structure of the organic electroluminescent device needs to be changed to accommodate to the mask-free full-color panels. Additionally, the different emission layers overlap with each other in the fabricated device structure, or emission materials with different-color light are mixed in an emission layer. For example, a blue emission layer overlaps over a red emission layer in U.S. Pat. No. 5,294,869, and a blue emission material is mixed in a red emission layer in U.S. Pat. No. 6,517,996. As a result, mixing light phenomenon may occur in the emission layer or in the device structure.
Therefore, it is very important to fabricate a mask-free organic electroluminescent panel without changing the original fabricating method of the RGB device structure and to fabricate the mask-free organic electroluminescent panel with low cost, high light purity and no light mixture.